Radio communications develop towards 3rd Generation (3G) and long term evolution (LTE) featuring high-speed and large-capacity data transmission. Thus, efficient data modulation and coding technologies such as quadrature phase shift keying (QPSK) and quadrature amplitude modulation (QAM) should be adopted to modulate the amplitude and phase of radio frequency (RF) carriers. In addition, the superposition of multiple carriers is performed. After the previous processing, RF signals often have a high peak to average power ratio (PAPR). For example, the PAPR of wideband code division multiple access (WCDMA) RF signals reaches over 6.5 dB. For the RF signals with a high PAPR, the efficiency is low if an RFPA is powered by a constant voltage source.
The efficiency can be improved significantly if the RFPA is powered by an envelope tracking (ET) power supply. The principle is: extracting an envelope signal from an RF signal, and adjusting the output voltage of the ET power supply with the envelope signal so that the RFPA works in a quasi-linear area close to a saturated area; and improving the efficiency of the RFPA significantly by using digital signal processing technologies such as digital pre-distortion. The bandwidth of an envelope signal is often over 1 MHz; therefore, the ET power supply should rapidly track the envelope signal, and high conversion efficiency is required.
The basic idea of a prior ET power supply for an RFPA is to divide a power supply into a linear power supply A and a switching power supply B, which are connected in parallel at the output ends to supply power to the RFPA. The linear power supply A outputs the power of the high frequency part, and the switching power supply B outputs the power of the low frequency part. The linear power supply A uses a closed voltage loop to track high frequency envelope signals and is a voltage controlled voltage source (VCVS). The switching power supply B uses a closed current loop to detect the output current of the linear power supply A and adjust the output current of the switching power supply B so that the output current of the linear power supply A is as low as possible. The switching power supply B is a current controlled current source (CCCS).
If the overall efficiency is expected to be high enough, the output power of the linear power supply A must be as low as possible and the output power of the switching power supply B must be as high as possible. The prerequisite is that the bandwidth of the switching power supply B is high enough and that sufficient power spectrums can be covered. Due to the limitations of factors such as power components in the actual design, there is a certain constraint between the output power, the switching frequency, and the efficiency. Currently, there is no independent switching power supply that can meet the conditions of high output power, high bandwidth, and high efficiency. Therefore, the overall efficiency in the prior art is not high.